Method Of Cooling Boil Off Gas And An Apparatus Therefor

ABSTRACT

A method of cooling a boil off gas stream ( 01 ) from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank ( 50 ) in a floating transportation vessel, said method comprising at least the steps of: compressing a boil off gas stream ( 01 ) from said liquefied cargo in two or more stages of compression comprising at least a first compression stage ( 65 ) and a final compression stage ( 75 ) to provide a compressed BOG discharge stream ( 06 ), wherein said first compression stage ( 65 ) has a first stage suction pressure and said final compression stage ( 75 ) has a final stage suction pressure; cooling the compressed BOG discharge stream ( 06 ) against one or more first coolant streams ( 202, 302 ) to provide a first cooled compressed BOG stream ( 08 ); providing a gaseous vent stream ( 51 ) from the first cooled compressed BOG stream ( 08 ); cooling the first cooled compressed BOG stream ( 08 ) against a second coolant stream ( 33 ) to provide a second cooled compressed BOG stream ( 35 ); expanding a portion of the second cooled compressed BOG stream ( 35 ) to the first stage suction pressure or below to provide a first expanded cooled BOG stream ( 33 ); using the first expanded cooled BOG stream ( 33 ) as the second coolant stream to provide a first expanded heated BOG stream ( 38 ); and cooling the gaseous vent stream ( 51 ) against the second coolant stream ( 33 ) to provide a cooled vent stream ( 53 ), wherein cooling of the first cooled compressed BOG stream ( 08 ) and cooling of the gaseous vent stream ( 51 ) occurs in a heat exchanger located adjacent to the liquefied cargo tank ( 50 ).

The present invention relates to methods for the cooling, particularly the re-liquefaction, of a boil off gas (BOG) from a liquefied cargo on a floating transportation vessel, and apparatus therefor.

Floating transportation vessels, such as liquefied gas carriers and barges, are capable of transporting a variety of cargoes in the liquefied state. In the present context, the liquefied cargo has a boiling point of greater than −110° C. when measured at 1 atmosphere, and includes ethane, liquefied petroleum gas, liquefied petrochemical gasses such as propylene and ethylene and liquefied ammonia.

One particular cargo is wholly or substantially ethane, generally being >90% ethane, or >95%, or >96%, or >97% or >98% or >99% ethane. Ethane is a useful product source for various industrial processes. Ethane can be extracted from natural gas production, fracking, or produced in the refining of crude oil. As a consequence, ethane may be associated with a plurality of other components, in particular methane. It is often desirable to liquefy ethane in a liquefaction facility at or near its source, as it can be stored and transported over long distances (generally in excess of normal pipeline distances) more readily as a liquid than in gaseous form because it occupies a smaller volume and may not need to be stored at high pressures.

The long distance transportation of a liquefied ethane cargo having a boiling point of about −87° C. when measured at 1 atmosphere may be carried out in a suitable liquefied gas carrier, such as an ocean-going tanker having one or more storage tanks to hold the liquefied ethane cargo. These storage tanks may be insulated and/or pressurized tanks. During the loading of the tanks and the storage of liquefied ethane cargo, gas may be produced due to the evaporation of the cargo. This evaporated cargo gas is known as boil off gas (BOG). In order to prevent the build up of BOG in the tank (with consequent pressure build up problems), a system may be provided on the carrier to re-liquefy the BOG so that it can be returned to the storage tank in a condensed state. This can be achieved by the compression and cooling of the BOG against a cold source. Ethane has a critical temperature of 32.18° C. at a pressure of 47.7 barg, such that seawater at a similar temperature would be unsuitable as the primary cold source. In many systems, the compressed BOG is cooled and condensed against a secondary refrigerant.

Liquefied petroleum gas is also a useful fuel source, such as for heating appliances and vehicles, as well as being a source of hydrocarbon compounds. LPG comprises one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylenes and ethane.

The long distance transportation of all such liquefied cargoes leads to the evaporation of the cargo known as boil off gas (BOG).

WO2012/143699A relates to a method and apparatus for re-liquefying a BOG stream from a liquefied cargo in a floating transportation vessel, said liquefied cargo having a boiling point of greater than −110° C. at 1 atmosphere, wherein a cooled vent stream is heat exchanged with a portion of the compressed, cooled and then expanded BOG stream. This is particularly suitable for liquefied cargos having boiling points of greater than −110° C. when measured at 1 atmosphere, but a need exists to provide an improved method of cooling, particularly re-liquefying as far as possible under reasonable OPEX and CAPEX, boil off gas from a liquefied ethane cargo, especially such cargoes comprising an increasing proportion of lighter components such as methane.

WO2016/027098A discloses an improved method and apparatus for re-liquefying a BOG stream from a liquefied ethane cargo in a floating transportation vessel.

The present invention provides improvements in methods and apparatus for re-liquefying a BOG stream in a floating transportation vessel.

In a first aspect, the present invention provides a method of cooling a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel, said method comprising at least the steps of:

-   -   compressing a boil off gas stream from said liquefied cargo in         two or more stages of compression comprising at least a first         compression stage and a final compression stage to provide a         compressed BOG discharge stream, wherein said first compression         stage has a first stage suction pressure and said final         compression stage has a final stage suction pressure;     -   cooling the compressed BOG discharge stream against one or more         first coolant streams to provide a first cooled compressed BOG         stream;     -   providing a gaseous vent stream from the first cooled compressed         BOG stream;     -   cooling the first cooled compressed BOG stream against a second         coolant stream to provide a second cooled compressed BOG stream;     -   expanding a portion of the second cooled compressed BOG stream         to the first stage suction pressure or below to provide a first         expanded cooled BOG stream;     -   using the first expanded cooled BOG stream as the second coolant         stream to provide a first expanded heated BOG stream; and     -   cooling the gaseous vent stream against the second coolant         stream to provide a cooled vent stream,     -   wherein cooling of the first cooled compressed BOG stream and         cooling of the gaseous vent stream occurs in a heat exchanger         located adjacent to the liquefied cargo tank.

In this way, not only is the first expanded cooled BOG stream used as the second coolant stream in a heat exchange/exchanger against the first cooled compressed BOG stream, but the cooling occurs in a heat exchanger located adjacent to the liquefied cargo tank to minimise the heat transfer dissipation thereinbetween.

The liquefied cargo is one of the group comprising: ethane, liquefied petroleum gas, liquefied petrochemical gas such as propylene and ethylene, and liquefied ammonia.

The term “having a boiling point of greater than −110° C. when measured at 1 atmosphere” for a cargo is used herein to mean said cargo having a boiling point that is higher than, i.e. numerically higher than, than minus 110° C. when measured at 1 atmosphere. For example, ethane has a boiling point of about −87° C. when measured at 1 atmosphere, and LPG has a boiling point of about −42° C. when measured at 1 atmosphere.

As used herein, the term “adjacent to” can be defined as being next to or neighbouring. This may refer to an entity which is nearby or within close enough proximity to another distinct entity such that both entities are touching, or attached. In the present invention, the liquefied cargo tank is located adjacent to the heat exchanger. The heat exchanger may be situated on top of the tank or close to it. Optionally, the heat exchanger may be touching the tank. Optionally, the heat exchanger may be attached to the tank.

As such, the term “heat exchanger located adjacent to the liquefied cargo tank” includes but is not limited to the heat exchanger being in contact with the liquefied cargo tank, but includes being close to liquefied cargo tank, or being closer to the liquefied cargo tank than the compressors or the cooling of the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream.

The terms “first”, “second”, “third”, “fourth”, etc. as used herein are intended to indicate a connection or relationship, which may or may not be a direct sequence except where explicitly stated. That is, there may be one or more other steps or processes or locations between a “second” and “third” feature. The terms are used to clarify a different nature or presence of an associated feature in or of a stream, and the present invention is not limited by these terms.

For the avoidance of doubt, the second coolant stream (i.e. first expanded cooled BOG stream) is at a lower temperature than the first cooled compressed BOG stream.

According to another embodiment, the step of cooling of the first cooled compressed BOG stream and cooling of the gaseous vent stream may take place in the same heat exchanger.

According to another aspect of the present invention, there is provided a method of cooling a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel, said method comprising at least the steps of:

-   -   compressing a boil off gas stream from said liquefied cargo in         two or more stages of compression comprising at least a first         compression stage and a second compression stage to provide a         compressed BOG discharge stream, wherein said first compression         stage has a first stage suction pressure and a first stage         discharge pressure, and said second compression stage has a         second stage suction pressure;     -   cooling the compressed BOG discharge stream against one or more         first coolant streams at a first cooling stage to provide a         first cooled compressed BOG stream;     -   cooling the first cooled compressed BOG stream against a second         coolant stream at a second cooling stage to provide a second         cooled compressed BOG stream;     -   cooling the second cooled compressed BOG stream against a third         coolant stream at a third cooling stage to provide a third         cooled compressed BOG stream;

providing a gaseous vent stream from the first cooled compressed BOG stream;

-   -   cooling the gaseous vent stream against the second coolant         stream at the second cooling stage to provide a cooled vent         stream;     -   cooling the cooled vent stream against the third coolant stream         at the third cooling stage to provide a cooled vent stream;     -   expanding a portion of the second cooled compressed BOG stream         at the second cooling stage to a pressure between that of the         first stage discharge pressure and the second stage suction         pressure to provide a first expanded cooled BOG stream;     -   expanding a portion of the third cooled compressed BOG stream at         the third cooling stage to the first stage suction pressure or         below to provide a second expanded cooled BOG stream;     -   using the first expanded cooled BOG stream as the second coolant         stream to provide a first expanded heated BOG stream; and     -   using the second expanded cooled BOG stream as the third coolant         stream to provide a second expanded heated BOG stream.

In this way, the portion of the second cooled compressed BOG stream at a pressure between that of the first stage discharge pressure and the second stage suction pressure is returnable into the compression at an intermediate pressure, and the portion of the third cooled compressed BOG stream at a first stage suction pressure or below is returnable into the compression at an initial pressure. The skilled man can now tune the cooling requirements of the BOG using different pressure expansions to achieve the most efficient cooling regime.

According to an embodiment of the present invention, the step of cooling the compressed BOG discharge stream against one or more first coolant streams to provide a first cooled compressed BOG stream may comprise:

-   -   pre-cooling the compressed BOG discharge stream against a         pre-cooling coolant stream as a first coolant stream to provide         a pre-cooled compressed BOG stream; and cooling the pre-cooled         compressed BOG stream against a first refrigerant stream as a         first coolant stream to provide the first cooled compressed BOG         stream.

That is, a pre-cooling coolant stream is used as one of the one or more first coolant streams in a heat exchange/exchanger against the compressed BOG discharge stream, which heat exchange/exchanger provides a pre-cooled compressed BOG stream and a heated pre-cooling coolant stream as a heated first coolant stream. The pre-cooling coolant stream may be part of an open pre-cooling coolant system or a closed pre-cooling coolant system. The pre-cooling coolant stream may be selected from a water stream, an air stream or a pre-cooling refrigerant stream, with a water or air stream being preferred. Typically if an open pre-cooling coolant circuit is used, the pre-cooling coolant stream may be selected from a seawater stream and an ambient air stream. Typically, if a closed pre-cooling coolant circuit is used, the pre-cooling coolant stream may be selected from a pre-cooling refrigerant stream. The cooling of the pre-cooled compressed discharge stream against the pre-cooling coolant stream can be carried out in a pre-cooling heat exchanger such as a shell and tube heat exchanger or a plate heat exchanger.

According to another embodiment of the present invention, the one or more first coolant streams comprise a first refrigerant stream, such as a first refrigerant comprising a single refrigerant or mixture of refrigerants. The first refrigerant should be capable of condensing the liquefied cargo (i) at the discharge pressure of the compression system and the discharge temperature of the compression system or (ii) at the discharge pressure of the compression system and the temperature of the pre-cooled compressed BOG stream. The first refrigerant may comprise one or more organic compounds, ammonia, and particularly hydrocarbons and fluorinated hydrocarbons such as propane, propylene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A.

According to another embodiment of the present invention, the cooling of the compressed BOG discharge stream or the pre-cooled compressed discharge stream against the first refrigerant stream is carried out in a discharge heat exchanger such as a shell and tube heat exchanger, a plate heat exchanger or an economiser.

According to another embodiment of the present invention all the compressed BOG discharge stream is cooled against the one or more first coolant streams.

In one embodiment of the present invention, the liquefied cargo is ethane, and the ethane comprises >0.1 mol % methane. Such the liquefied ethane cargo may comprise >0.4 mol % methane, including >0.5 mol %, 0.6 mol %, >0.7 mol %, >0.8 mol %, >0.9 mol % and >1.0 mol % methane. The present invention extends to a liquefied ethane cargo having 1-5 mol % methane, optionally >5 mol % methane.

The number of stages of compression is not a limiting factor of the present invention. Optionally, the method comprises three or four stages of compression.

Optionally, it is desired to provide a fully condensed boil off gas as the first cooled compressed BOG stream, but the present invention extends to a method wherein the boil off gas is not fully condensed after cooling against the one or more first coolant streams.

The present invention overcomes the difficulty of using certain types of heat exchange, in particular certain types of heat exchanger, and more particularly conventional shell & coil economisers, where the temperature approach is limited by the composition of the fluid in the shell. Where the composition of the fluid in the shell may be a single component, i.e. a sufficiently ‘pure’ gas, its cooling against an expanded portion of the compressed BOG is well known and extensive. However, this cooling duty is reduced in a multi-component mixture, and is dramatically reduced in a multi-component mixture having a significant difference in boiling points, such as in particular ethane and methane. Thus, the present invention improves the coefficient of performance of the cooling cycle of a liquefied ethane cargo comprising a significant methane amount, i.e. the present invention improves the coefficient of performance of cargo currently considered de minimis (e.g. 0.1 mol % or less methane), and allows operation with cargoes comprising much higher methane contents (e.g. about or above 0.4 or 0.5 mol % methane).

The present invention also seeks to maintain the use of current onboard equipment and apparatus with its known OPEX and CAPEX, rather than seeking to introduce and work out how to use new equipment with new operating requirements.

Thus, according to another embodiment of the present invention, the cooling of the first cooled compressed BOG stream against the second coolant stream is carried out in a heat exchanger.

According to another embodiment of the present invention all the first cooled compressed BOG stream is cooled against the second coolant stream.

According to another embodiment of the present invention all the gaseous vent stream is cooled against the second coolant stream.

In some embodiments of the present invention, the method comprises or further comprises the steps of:

-   -   providing a gaseous vent stream from the first cooled compressed         BOG stream;     -   expanding a portion of the second cooled compressed BOG stream         to provide a first expanded cooled BOG stream;     -   using the first expanded cooled BOG stream as the second coolant         stream to cool the gaseous vent stream to provide a cooled vent         stream.

In this way, the present invention can further provide increased re-liquefying of previously considered ‘non-condensables’ or ‘non-condensing’ components in the compressed BOG.

Optionally, the method of the present invention comprising the further step of separating the cooled vent stream to provide a vent discharge stream and a cooled vent BOG return stream.

Optionally, the method of the present invention comprising the further steps of expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream, and passing the expanded cooled vent BOG return stream to a storage tank.

Optionally, the method comprises the further steps of:

-   -   expanding the cooled vent BOG return stream to provide an         expanded cooled vent BOG return stream;     -   expanding the cooled vent discharge stream to provide an         expanded cooled vent discharge stream;     -   passing the heat exchanged vent BOG return stream and the         expanded cooled vent discharge stream to a storage tank.

Optionally, the stages of compression are the compression stages of a multi-stage compressor.

The first cooled compressed BOG stream is cooled against at least one second coolant stream to provide a second cooled compressed BOG stream. A portion of the second cooled compressed BOG stream is expanded to the first stage suction pressure or below to provide a first expanded cooled BOG stream. Preferably, the first expanded cooled BOG stream is used as a second coolant stream, to provide a first expanded heated BOG stream.

Optionally, the first expanded cooled BOG stream used as the second coolant stream comprises both liquid and gas phases. That is, it does not need to be separated into separate gas and liquid phases prior to use as a second coolant stream.

According to another aspect of the present invention, the present invention provides a method of cooling a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel, said method comprising at least the steps of:

-   -   compressing a boil off gas stream from said liquefied cargo in a         compression system comprising two or more compression trains,         each train comprising two or more stages of compression         comprising at least a first compression stage and a second         compression stage to provide a compressed BOG discharge stream,         wherein said first compression stage has a first stage suction         pressure and a first stage discharge pressure, and said second         compression stage has a second stage suction pressure;     -   cooling the compressed BOG discharge stream against one or more         first coolant streams at a first cooling stage to provide a         first cooled compressed BOG stream;     -   cooling the first cooled compressed BOG stream against a second         coolant stream having a pressure between that of the first stage         discharge pressure and the second stage suction pressure at a         second cooling stage to provide a second cooled compressed BOG         stream;     -   cooling the second cooled compressed BOG stream against a third         coolant stream having first stage suction pressure or below at a         third cooling stage to provide a third cooled compressed BOG         stream;     -   providing a gaseous vent stream from the first cooled compressed         BOG stream;     -   cooling the gaseous vent stream against the second coolant         stream at the second cooling stage to provide a cooled vent         stream;     -   cooling the cooled vent stream against the third coolant stream         at the third cooling stage to provide a cooled vent stream;     -   expanding a portion of the second cooled compressed BOG stream         at the second cooling stage to a pressure between that of the         first stage discharge pressure and the second stage suction         pressure to provide a first expanded cooled BOG stream;     -   expanding a portion of the third cooled compressed BOG stream at         the third cooling stage to the first stage suction pressure or         below to provide a second expanded cooled BOG stream;     -   using the first expanded cooled BOG stream as the second coolant         stream to provide a first expanded heated BOG stream;     -   using the second expanded cooled BOG stream as the third coolant         stream to provide a second expanded heated BOG stream; and     -   passing the first expanded heated BOG stream and the second         expanded heated BOG stream into one or more of the two or more         compression trains at either a pressure between that of the         first stage discharge pressure and the second stage suction         pressure, or a pressure at or below the first stage suction         pressure, or both.

In this way, skilled man can further tune how the portions of the second and third cooled compressed BOG streams are returnable into the same or different compression trains at different compression stages using different pressure expansions, to achieve the most efficient cooling regime.

Optionally, the method further comprises passing the first expanded heating BOG stream from the second cooling stage into second compression train, and expanding the third coolant stream after the third cooling stage to the first stage suction pressure or below and passing the second expanded heated BOG stream into a first compression train.

Alternatively or additionally, the method further comprises expanding at least a portion of the second coolant stream to a pressure between that of the first stage discharge pressure and the second stage suction pressure and passing the first expanded heated BOG stream into a first compression train, and passing at least a portion of the third coolant stream after the third cooling stage into a second compression train.

Optionally, the method comprises using a controller to control the passage of the various streams into the compression trains at the same or different compression stages.

For example, a controller could control the passage of the first expanded heated BOG stream from the second cooling stage and the second expanded heated BOG stream after the third cooling stage into the first compression train and the second compression train.

Or a controller could control the passage of the first expanded heated BOG stream from the second cooling stage into the first compression train or the second compression train or both.

Or a controller could control the passage of the second expanded heated BOG stream after the third cooling stage into the first compression train or the second compression train or both.

The skilled reader can see than variations of the above embodiments and examples are possible to improve the heat/energy balance required to cool a boil off gas stream from a liquefied cargo in a floating transportation vessel.

According to another aspect of the present invention, there is provided an apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel comprising at least:

-   -   a compression system to compress the boil off gas stream         comprising two or more stages of compression comprising at least         a first compression stage and a final compression stage to         provide a compressed BOG discharge stream, one or more first         heat exchangers to cool the compressed BOG discharge stream to         provide a first cooled compressed BOG stream;     -   one or more second heat exchangers adjacent to the liquefied         cargo tank to further cool the first cooled compressed BOG         stream and a gaseous vent stream against a coolant stream to         provide a second cooled compressed BOG stream and a cooled vent         stream.

According to another aspect of the present invention, there is provided an apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel comprising a plurality of components, said apparatus comprising at least:

-   -   a compression system to compress the boil off gas stream         comprising two or more compression trains, each train comprising         two or more stages of compression comprising at least a first         compression stage and a second compression stage to provide a         compressed BOG discharge stream, wherein said first compression         stage has a first stage suction pressure and a first stage         discharge pressure, and said second compression stage has a         second stage suction pressure;     -   a first cooling stage comprising one or more first heat         exchangers to cool the compressed BOG discharge stream to         provide a first cooled compressed BOG stream;     -   a second cooling stage comprising one or more second heat         exchangers to cool the first cooled compressed BOG stream to         provide a second cooled compressed BOG stream using a second         coolant stream having a pressure between that of the first stage         discharge pressure and the second stage suction pressure; and     -   a third cooling stage comprising one or more third heat         exchangers to cool the second cooled compressed BOG stream to         provide a third cooled compressed BOG stream using a third         coolant stream having first stage suction pressure or below.

Optionally, the apparatus may further comprise a first passage to pass the first expanded heated BOG stream once used in the second cooling stage into a second compression train, and a second passage to expand the third coolant stream after its passage through the third cooling stage to the first stage suction pressure or below and to pass the second expanded heated BOG stream into a first compression train.

Optionally, the apparatus may further comprise a third passage to expand at least a portion of the second coolant stream after its passage through the second cooling stage to a pressure between that of the first stage discharge pressure and the second stage suction pressure and to pass the first expanded heated BOG stream into a first compression train, and a fourth passage to pass at least a portion of the third coolant stream after its passage through the third cooling stage into a second compression train.

Optionally, the apparatus may further comprise a controller to control the passage of the first expanded heated BOG stream from the second cooling stage and the second expanded heated BOG stream after the third cooling stage into the first compression train and the second compression train.

Optionally, the apparatus may further comprise a controller able to control the passage of the first expanded heated BOG stream from the second cooling stage into the first compression train or the second compression train or both, and to control the passage of the second expanded heated BOG stream after the third cooling stage into the first compression train or the second compression train or both.

The apparatus of the present invention, optionally using the controller, allow the skilled man to tune how the portions of the second and third cooled compressed BOG streams are returnable into the same or different compression trains at different compression stages using different pressure expansions, to achieve the most efficient cooling regime.

Optionally, the apparatus as defined herein is operable using the method as defined herein.

Optionally, a further heat exchanger or separator could be introduced to the system after the compression stage, depending on the requirements of the system to handle products other than ethane, such as liquefied cargoes having a a boiling point of greater than −110° C. at 1 atmosphere and comprising a plurality of components.

According to a further aspect of the present invention, there is provided a floating transportation vessel for a liquefied cargo having apparatus as defined herein or operating a method as defined herein.

The present invention is applicable to any floating transportation vessel for a liquefied cargo. The present invention may be utilized in floating transportation vessels where the liquefied cargo storage tanks are fully refrigerated to maintain the cargo in liquid phase at approximately atmospheric pressure by lowering the temperature, as well as in those vessels in which the cargo in the storage tanks is maintained in the liquid phase by a combination of reduced temperature and increased pressure versus ambient.

In order to obtain the benefits of a method and apparatus disclosed herein, the use of economizers is not essential. However, in certain embodiments, heat exchangers such as economizers can be placed between consecutive stages of compression, such as between the first and second stages, to cool the intermediate compressed BOG streams. Where three or more stages of compression are present, heat exchangers, such as economizers or intercoolers, such as seawater intercoolers, to allow the cooling of an intermediate compressed BOG streams may be provided between the second and final stages of compression.

For instance, an intercooler can be situated between the second and third stages of compression. Alternatively, an economizer can be situated between the second and third, as well as between the first and second stages of compression. In an economizer, an expanded, optionally further cooled, portion of the cooled compressed BOG stream can be heat exchanged with an intermediate compressed BOG stream. In a further embodiment, an expanded, optionally further cooled, portion of the cooled compressed BOG stream can be heat exchanged with an optionally further cooled portion of the cooled compressed discharge stream. This leads to further improvements in the coefficient of performance and increased cooling, particularly re-liquefaction, capacity.

It will be apparent that the method and apparatus disclosed herein can be applied to an existing floating transportation vessel as a retro-fit, by maintaining the number of stages of compression present and adding the necessary piping, valves and controls to carry out the cooling of a second cooled compressed BOG stream against an expanded portion of the third cooled BOG stream.

As used herein, the term “multiple stages of compression” defines two or more stages of compression in series in a compression system. Each stage of compression may be achieved by one or more compressors. The one or more compressors of each compression stage may be independent from those of the other stages of compression, such that they are driven separately. Alternatively, two or more of the stages of compression may utilize compressors which are linked, typically powered by a single driver and drive shaft, with optional gearing. Such linked compression stages may be part of a multi-stage compressor, also referred herein as a ‘compression train’.

As used herein, the term “controller” defines any device capable of directing the coolant streams into various compression trains. Said device may be operable using known technology. The controller is capable of detecting stream pressure. Additionally, the controller is capable of directing streams into certain compression trains. The controller is capable of organising a plurality of streams simultaneously. The controller is operable with a plurality of compression trains.

The method and apparatus disclosed herein requires at least two stages of compression. After the first stage of compression, each subsequent stage provides an increased pressure compared to the pressure at the discharge of a previous stage. The term “consecutive stages” refers to pairs of adjacent stages of compression i.e. a stage (n) and the next (n+1) stage where ‘n’ is a whole number greater than 0. Consequently, consecutive stages are, for instance, first and second stages or second and third stages or third and fourth stages. Intermediate compressed streams (and cooled intermediate compressed streams) refer to those streams connecting consecutive stages of compression. The terms “next stage of compression” or “subsequent stage of compression” used in relation to the cooled intermediate compressed stream refer to the numerically higher number (and higher pressure stage) of the two consecutive stages defining the intermediate stream.

The heat exchange steps may be indirect, where the two or more streams involved in the heat exchange are separated and not in direct contact. Alternatively, the heat exchange may be direct, in which case the two or more streams involved in the heat exchange can be mixed, thereby producing a combined stream.

Embodiments of the invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings in which:

FIG. 1 shows a schematic diagram of a system of cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to one embodiment of the invention.

FIG. 2 shows a schematic diagram of a system for cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to another embodiment of the invention.

FIG. 3 shows a schematic diagram of a system for cooling, particularly re-liquefying, boil off gas from a liquefied cargo in a floating transportation vessel according to another embodiment of the invention.

Floating re-liquefaction systems draw the vapor, also known as boil off gas, from one or more storage tanks and pass the boil off gas to a compressor in which it is compressed such that the compressed vapor can be cooled and condensed against one or more coolants as the heat sink/refrigerant. For instance, seawater may be used to pre-cool, typically de-superheat, the compressed vapour in an open cycle pre-cooling circuit. The pre-cooled compressed vapour can then be further cooled against a refrigerant in a closed cycle refrigerant circuit.

The method and apparatus disclosed herein seeks to provide an improved method and apparatus of re-liquefying BOG. An embodiment of the method and apparatus according to the present invention is disclosed in FIG. 1. Where appropriate, identical stream and component names, and the same reference numerals as those in FIG. 1 have been used for corresponding streams and components in the remaining Figures.

FIG. 1 shows a liquefied cargo storage tank (50) in a floating transportation vessel, such as an ethane or LPG carrier. A liquefied ethane cargo may comprise other components, such as a proportion of methane.

In order to cool, particularly re-liquefy, evaporated cargo from the storage tank (50), a boil off gas stream (01), comprising evaporated cargo, is passed to a compression system (60) having two or more stages of compression. The boil off gas stream (01) may have a pressure (the “BOG pressure”) in the range of from above 0 to 500 kPa gauge. The compression system (60) may be a multi-stage compressor comprising two or more stages. By “multi-stage compressor” it is meant that each compression stage in the compressor is driven by the same drive shaft, sometimes termed a compression train. Alternatively, the compression system (60) may comprise independently driven compressors for each of the stages of compression. When the compression system (60) is a multi-stage compressor, it is typically a reciprocating compressor.

The embodiment of FIG. 1 shows a compression system (60) having a first stage (65) and a second stage (70) and a third and final stage (75), although the method and apparatus described herein is also applicable to compressors having two stages or more than three stages. The first stage (65) and final stage (75) of compression provide low and high pressure streams respectively at their discharge.

The compression system (60) compresses the boil off gas stream (01) to provide a compressed BOG discharge stream (06). The compressed BOG discharge stream (06) may have a pressure (the “final stage pressure”) in the range of from 1.5 to 3.2 MPa or above, eg. up to 6 MPa.

The compressed BOG discharge stream (06) is cooled in one or more first heat exchangers (200, 300) against one or more first coolant streams (202, 302) to provide first cooled compressed BOG stream (08).

In the embodiment of FIG. 1, the compressed BOG discharge stream (06) can be passed to a pre-cooling heat exchanger (200) as one of the one or more first heat exchangers. The compressed BOG discharge stream (06) is pre-cooled against a pre-cooling coolant stream as one of the one of more first coolant streams. The pre-cooling coolant stream (202) may be an air or a water stream, such as an ambient air or seawater stream. The pre-cooling heat exchanger (200) may be a shell and tube heat exchanger or a plate heat exchanger. The pre-cooling heat exchanger may de-superheat the compressed BOG discharge stream (06). The pre-cooling heat exchanger (200) provides a pre-cooled compressed BOG stream (07) and heated pre-cooling coolant stream (204). Typically, the seawater used as the pre-cooling coolant would have a temperature of +36° C. or below, more typically +32° C. or below.

The pre-cooling heat exchange/exchanger (200) is optional in the method and apparatus disclosed herein. It is advantageous because it reduces the cooling duty of the subsequent cooling steps. However, is it not an essential aspect, such that in an alternative embodiment, the compressed BOG discharge stream (06) can be passed directly to the discharge heat exchanger (300). In such circumstances, the cooling capacity of the discharge heat exchanger (300) would have to be increased to compensate for the absence of pre-cooling.

The pre-cooled compressed BOG stream (07) can then be passed to a discharge heat exchanger (300) as another of the one or more first heat exchangers. The discharge heat exchanger (300) cools the pre-cooled compressed BOG stream (07) against a first refrigerant stream (302) as another of the one or more first coolant streams. The discharge heat exchanger (300) provides a first cooled compressed BOG stream (08) and a heated first refrigerant stream (304).

The first refrigerant stream (302), discharge heat exchanger (300) and heated first refrigerant stream (304) may be part of a first refrigerant system (not shown). Such a first refrigerant system may further comprise a first refrigerant compressor to compress the heated first refrigerant stream (304) to provide a compressed first refrigerant stream, a first refrigerant cooler to cool the first refrigerant to provide a cooled compressed first refrigerant stream and a first refrigerant expansion device to expand the cooled compressed first refrigerant stream to provide the first refrigerant stream (302). The first refrigerant system may be a closed system. The first refrigerant may comprise one or more organic compounds, particularly hydrocarbons and fluorinated hydrocarbons such as propane, propylene, difluoromethane and pentafluoromethane, including the fluorinated hydrocarbon mixture R-410A, as well as one or more inorganic compounds such as ammonia.

Where the cooled compressed BOG stream (08) is not fully condensed, there is a gaseous vent stream also provided, either from the discharge heat exchanger (300) as stream (51 a), and/or from the discharge receiver (305) as stream (51 b). Whilst FIG. 1 shows the two streams (51 a, 51 b) as separate, such streams may be provided separately or combined or without any distinction, depending upon the nature and construction of the discharge heat exchanger (300) and the discharge receiver (305). The provision of these stream or streams is known in the art. The gaseous vent stream (51) may comprise both ‘non-condensable’ components and ‘in-condensable’ components. The in-condensable components are generally considered to be components which cannot practically ever by compressed and condensed within the confines and operating parameters of a particular floating transportation vessel BOG cooling system, and primarily relate to nitrogen.

The first cooled compressed BOG stream (08) is then second cooled. This can be achieved by passing the first cooled compressed BOG stream (08) to a heat exchanger (20). The cooling of the first cooled compressed BOG stream (08) is against a second coolant stream to provide a second cooled compressed BOG stream (35). The action of the second coolant is to provide a second cooled compressed BOG stream (35). A portion of this stream (35) is expanded to the first stage suction pressure or below to provide a first expanded cooled BOG stream (33).

The cooling of the gaseous vent stream (51) using the same heat exchanger (20) can condense a portion of the components of the boil off gas which could not be condensed in the discharge heat exchanger (300) against the first refrigerant such as propane or propylene. The so formed cooled vent stream (53) is typically an at least partly condensed stream.

It is a particular feature of the embodiment of the present invention that cooling of the first cooled compressed BOG stream (08) and cooling of the gaseous vent stream (51) takes place in a heat exchanger (20), which is located adjacent to the liquefied cargo tank (50). In a conventional system, heat exchangers are typically located adjacent to the compressor(s), with the storage tank located remotely and connected via long pipelines. Locating the heat exchanger adjacent to the cargo tank in the present invention offers an improvement in performance by reducing the heat transfer from the surroundings (which are significantly warmer than the returning cooled BOG) into the BOG stream (11). The reduced heat transfer has the effect of improving overall performance in two ways—less cooling is ‘lost’ to the surroundings and the accumulation of methane in the BOG vapour is reduced.

In the arrangement shown in FIG. 1, the heat exchanger (20) to cool the first cooled compressed BOG stream (08) and the gaseous vent stream (51) is located closer to the liquefied cargo tank (50) than the compressors (60) and the cooling of the compressed BOG discharge stream (06) against one or more first coolant streams to provide a first cooled compressed BOG stream (08). In this way, the efficiency of the overall arrangement shown in FIG. 1 can reduce the heat loss of the ‘colder’ parts of the arrangement, compared to those parts wherein the BOG is at a relative higher temperature.

FIG. 1 also shows the cooled vent stream (53) optionally being passed to a vent stream separator (150), such as a gas/liquid separator. The vent stream separator (150) provides a vent discharge stream (55) being wholly or substantially the in-condensable components, which is typically a vapour stream, and a cooled vent BOG return stream (57), which is typically a condensed stream, comprising those components of the boil off gas which were condensed in the heat exchanger. The pressure of the vent discharge stream (55) may be reduced, for instance to a pressure appropriate for return to the storage tank (50), for storage elsewhere or for venting.

Optionally, separator (150) could be a discrete separator device, or be integrated into the tank, or utilise an existing feature of the tank such as the “emergency pump removal column”.

The cooled vent BOG return stream (57) may be passed through a vent return stream pressure reduction device (58), such as a Joule-Thomson valve or expander, to provide an expanded cooled vent BOG return stream (59). The expanded cooled vent BOG return stream (59) can be passed to the storage tank (50), for instance by addition to the expanded cooled BOG return stream (36).

FIG. 2 shows another method and apparatus of the present invention. In common with FIG. 1, FIG. 2 shows a liquefied cargo storage tank (50) from which a boil off gas stream (01), comprising evaporated cargo, is passed to a compression system (60), having three stages of compression being a first stage (65), a second and intermediate stage (70) and a third and final stage (75).

The compression system (60) provides a compressed BOG discharge stream (06) which can be cooled against one or more first coolant streams (202, 302) at a first cooling stage (400) to provide a first cooled compressed BOG stream (08). The first cooled compressed stream (08) is then cooled against a second coolant stream at a second cooling stage (410) to provide a second cooled compressed BOG stream (34). Stream (34) is then cooled against a third coolant stream at a third cooling stage (42) to provide a third cooled compressed BOG stream (35).

A gaseous vent stream (51) is provided from the first cooled compressed BOG stream (08) in the same way as shown in FIG. 1, and cooled against the second coolant stream at the second cooling stage (410) to provide a cooled vent stream (52). The cooled vent stream (52) is then cooled against the third coolant stream at the third cooling stage (420) to provide a cooled vent stream (53).

In FIG. 2, a portion of the second cooled compressed BOG stream (34) is expanded to a pressure between that of the first stage discharge pressure and the second stage suction pressure at the second cooling stage (410) to provide a first expanded cooled BOG stream (34 a).

Meanwhile, a portion of the third cooled compressed BOG stream (35) is expanded to the first stage suction pressure or below at the third cooling stage (420) to provide a second expanded cooled BOG stream (35 a).

The first expanded cooled BOG stream (34 a) is then used as the second coolant stream to provide a first expanded heated BOG stream (39), which can be returned into the compression system (60) between the first stage (65) and the second stage (70).

And the second expanded cooled BOG stream (35 a) is then used as the third coolant stream to provide a second expanded heated BOG stream (38), which can be returned into the compression system (60) before be first stage (65).

Overall, the user can then tune the amounts of the first and second expanded cooled BOG streams (34 a, 35) to improve, preferably to maximise, the overall efficiency of the cooling configuration, and so to reduce the energy consumption required, which may vary over time.

FIG. 3 shows another method and apparatus of the present invention.

FIG. 3 shows a liquefied cargo storage tank (50) from which a boil off gas stream (01), comprising evaporated cargo, is passed to a compression system (60) comprising two or more compression trains (60 a, 60 b), wherein each train comprises two or more stages of compression. Each train comprises at least a first compression stage (65, 65′) and a second compression stage (70,70′) to provide a combined compressed BOG discharge stream (06).

The compressed BOG discharge stream (06) is cooled in one or more first heat exchangers (200, 300) to provide a first cooled compressed BOG stream (08) as described hereinbefore.

The first cooled compressed BOG stream (08) is cooled against a second coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure at a second cooling stage (410) to provide a second cooled compressed BOG stream (34). The second cooled compressed BOG stream (34) is cooled against a third coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure at a third cooling stage (420) to provide a third cooled compressed BOG stream (35).

Optionally, a gaseous vent stream (51) is provided from the first cooled compressed BOG stream (08) as described hereinbefore, and is also cooled against the second coolant stream at the second cooling stage (410) to provide a cooled vent stream (52). The cooled vent stream (52) is cooled against the third coolant stream at the third cooling stage (420) to provide a cooled vent stream (53).

A portion of the second cooled compressed BOG stream (34) is expanded at the second cooling stage (410) to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide a first expanded cooled BOG stream (34 a). The first expanded cooled BOG stream (34 a) is then used as the second coolant stream to provide a first expanded heated BOG stream (39).

Meanwhile, a portion of the third cooled compressed BOG stream (35) is expanded at the third cooling stage (420) to provide a second expanded cooled BOG stream (35 a). The second expanded cooled BOG stream (35 a) is used as the third coolant stream to provide a second expanded heated BOG stream (38).

The first expanded heated BOG stream (39) and the second expanded heated BOG stream (38) are passed into one or more of the two or more compression trains (60 a, 60 b), at either a pressure between that of the first stage discharge pressure and the second stage suction pressure, or a pressure at or below the first stage suction pressure, or both. These streams can be passed along passages and through valves (22) to achieve the desired expanded pressure for delivery into the desired part of the compression system (60) to improve, preferably to maximise, the efficiency of the cooling configuration.

In one example, the first expanded heated BOG stream (39) from the second cooling stage (410) can be passed into the second compression train (60 b), and the third coolant stream (35 a) can be expanded after the third cooling stage (420) to the first stage suction pressure or below to provide the second expanded heated BOG stream (38), which may be passed into the first compression train (60 a).

In another example, at least a portion of the second coolant stream (34 a) may be expanded after the second cooling stage (410) to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide the first expanded heated BOG stream (39), which may be passed into the first compression train (60 a), and at least a portion of the third coolant stream (35 a) after the third cooling stage (420) may be passed into the second compression train (60 b).

The passing of the coolant streams into the various compression trains can be organised by a controller (not shown), able to operate the required valves to balance the required flows.

Optionally, a number of identical sets of compressors and heat exchangers are provided. OPEX & CAPEX benefit can be obtained by operating the compressor systems in an integrated fashion via the provision of valves or gates to direct the cooled BOG flow appropriately. The units can operate independently if required, depending on the cooling capacity required.

The person skilled in the art will understand that the invention can be carried out in many various ways without departing from the scope of the appended claims. For instance, the invention encompasses the combination of one or more of the optional or preferred features disclosed herein. 

1. (canceled)
 2. (canceled)
 3. A method of cooling a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a floating transportation vessel, said method comprising the steps of: compressing a boil off gas stream from said liquefied cargo in at least two stages of compression comprising at least a first compression stage and a second compression stage to provide a compressed BOG discharge stream, wherein said first compression stage has a first stage suction pressure and a first stage discharge pressure, and said second compression stage has a second stage suction pressure; cooling the compressed BOG discharge stream against at least one first coolant streams at a first cooling stage to provide a first cooled compressed BOG stream; cooling the first cooled compressed BOG stream against a second coolant stream at a second cooling stage to provide a second cooled compressed BOG stream; cooling the second cooled compressed BOG stream against a third coolant stream at a third cooling stage to provide a third cooled compressed BOG stream; providing a gaseous vent stream from the first cooled compressed BOG stream; cooling the gaseous vent stream against the second coolant stream at the second cooling stage to provide a cooled vent stream; cooling the cooled vent stream against the third coolant stream at the third cooling stage to provide a cooled vent stream; expanding a portion of the second cooled compressed BOG stream at the second cooling stage to a pressure between that of the first stage discharge pressure and the second stage suction pressure to provide a first expanded cooled BOG stream; expanding a portion of the third cooled compressed BOG stream at the third cooling stage to the first stage suction pressure or below to provide a second expanded cooled BOG stream; using the first expanded cooled BOG stream as the second coolant stream to provide a first expanded heated BOG stream; and using the second expanded cooled BOG stream as the third coolant stream to provide a second expanded heated BOG stream.
 4. The method as in claim 3, wherein the liquefied cargo is selected from the group comprising: ethane, liquefied petroleum gas, liquefied petrochemical gas such as propylene and ethylene, and liquefied ammonia.
 5. The method as in claim 3 wherein the liquefied cargo is ethane.
 6. The method as in claim 5 wherein the liquefied ethane cargo comprises >0.1 mol % methane.
 7. The method as in claim 6 wherein the liquefied ethane cargo comprises >0.5 mol % methane.
 8. The method as in claim 3 comprising at least three stages of compression.
 9. The method as in claim 3 comprising the further step of: separating the further cooled vent stream to provide a vent discharge stream and a cooled vent BOG return stream.
 10. The method as in claim 9 comprising the further steps of: expanding the cooled vent BOG return stream to provide an expanded cooled vent BOG return stream; and passing the expanded cooled vent BOG return stream to a storage tank.
 11. The method as in claim 3 wherein the step of cooling the compressed BOG discharge stream against at least one first coolant stream to provide a first cooled compressed BOG stream comprises: pre-cooling the compressed BOG discharge stream against a pre-cooling coolant stream as a first coolant stream to provide a pre-cooled compressed BOG stream; and cooling the pre-cooled compressed BOG stream against a first refrigerant stream as a first coolant stream to provide the first cooled compressed BOG stream.
 12. The method as in claim 11 wherein the pre-cooling coolant stream is selected from the group comprising: a seawater stream, an ambient air stream and a refrigerant stream.
 13. The method as in claim 11 wherein the first refrigerant stream is selected from the group comprising propane and propylene.
 14. The method as in claim 3 wherein stages of compression are the compression stages of a multi-stage compressor.
 15. An apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank in a floating transportation vessel, said apparatus comprising: a compression system to compress the boil off gas stream comprising at least two stages of compression comprising at least a first compression stage and a final compression stage to provide a compressed BOG discharge stream, at least one first heat exchanger to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; at least one second heat exchanger adjacent to the liquefied cargo tank to further cool the first cooled compressed BOG stream and a gaseous vent stream against a coolant stream to provide a second cooled compressed BOG stream and a cooled vent stream.
 16. An apparatus to cool a boil off gas stream from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere in a liquefied cargo tank in a floating transportation vessel comprising, the apparatus comprising: a compression system to compress the boil off gas stream comprising at least two compression trains, each train comprising at least two stages of compression comprising at least a first compression stage and a second compression stage to provide a compressed BOG discharge stream, wherein said first compression stage has a first stage suction pressure and a first stage discharge pressure, and said second compression stage has a second stage suction pressure; a first cooling stage comprising at least one first heat exchanger to cool the compressed BOG discharge stream to provide a first cooled compressed BOG stream; a second cooling stage comprising at least one second heat exchanger to cool the first cooled compressed BOG stream to provide a second cooled compressed BOG stream using a second coolant stream having a pressure between that of the first stage discharge pressure and the second stage suction pressure; and a third cooling stage comprising at least one third heat exchanger to cool the second cooled compressed BOG stream to provide a third cooled compressed BOG stream using a third coolant stream having first stage suction pressure or below.
 17. The apparatus as in claim 16 further comprising: a first passage to pass a first expanded heated BOG stream provided from the passage of the second coolant stream through the second cooling stage into a second compression train, and a second passage to expand a second expanded heated BOG stream provided from the passage of the third coolant stream through the third cooling stage to the first stage suction pressure or below, and to pass the further expanded heated BOG stream into a first compression train.
 18. The apparatus as in claim 17 further comprising: a third passage to expand at least a portion of a first expanded heated BOG stream provided from the passage of the second coolant stream through the second cooling stage, to a pressure between that of the first stage discharge pressure and the second stage suction pressure, and to pass the further expanded heated BOG stream into a first compression train, and a fourth passage to pass at least a portion of a second expanded heated BOG stream provided from the passage of the third coolant stream through the third cooling stage into a second compression train.
 19. (canceled)
 20. (canceled)
 21. The apparatus as in claim 20 further comprising: a controller able to control the passage of the first expanded heated BOG stream from the second cooling stage into the first compression train or the second compression train or both, and to control the passage of the second expanded heated BOG stream after the third cooling stage into the first compression train or the second compression train or both.
 22. (canceled)
 23. (canceled)
 24. (canceled)
 25. The method of cooling a boil of gas stream as in claim 3, wherein the cooling of the first cooled compressed BOG stream and cooling of the gaseous vent stream occurs in a heat exchanger located adjacent to a tank retaining the liquefied cargo.
 26. The method of cooling a boil of gas stream as in claim 25, wherein cooling of the first cooled compressed BOG stream and the cooling of the gaseous vent stream take place in the heat exchanger.
 27. A method of cooling a boil off gas stream as in claim 3 wherein the pressure of the second coolant stream is between that of the first stage discharge pressure and the second stage suction pressure at a second cooling stage within the step of cooling of the first cooled compressed BOG stream against a second coolant stream, and the third coolant stream has a first stage suction pressure within the step of cooling the second cooled compressed BOG stream against the third coolant stream the third cooling stage; and further comprising the step of passing the first expanded heated BOG stream and the second expanded heated BOG stream into at least one compression stage at a pressure that is between the first stage discharge pressure and the second stage suction pressure, that is at or below the first stage suction pressure, or that is between the first stage discharge pressure and the second stage suction pressure and also at or below the first stage suction pressure. 